Operating expenses (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Fuel [Line Items]
Fuel Power Costs	$ 307.9	$ 391.6	$ 401.7
Public Service Company Of New Hampshire [Member]
Fuel [Line Items]
Fuel Power Costs	115.9	184.3	174.1
Yankee Gas Services Company [Member]
Fuel [Line Items]
Fuel Power Costs	$ 191.3	$ 206.4	$ 226.1